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To what extent do our genes really dictate longevity?

A long-held belief has it that some people have "better genes" than others, which predisposes them to living longer lives. A large new study, however, questions the accuracy of this idea.

Do genes really predict lifespan?

Scientists from Calico Life Sciences, a research and development company — in collaboration with colleagues from Ancestry, an online genealogy resource — have recently analyzed data from millions of people to establish whether genetic makeup really does have a crucial say in longevity.

The study's lead author is Graham Ruby, who is affiliated with Calico Life Sciences.

Ruby and team studied the family trees of over 400 million people and found that genes have a lower impact on how long a person can expect to live than scientists had previously believed.

Their findings now appear in GENETICS, the journal of the Genetics Society of America.

The role played by genes may be minimal

The researchers used data from the Ancestry website and focused on heritability, which measures to what extent genetic specificities explain differences in people's individual traits.

They wanted to assess the heritability of human lifespan — that is, whether the fact that a person's parents were long-lived could predict that person's own lifespan.

Furthermore, the scientists wanted to see whether any predictions of longevity would rely predominantly on genetic makeup, or on other factors altogether.

"Partnering with Ancestry allowed this new study to gain deeper insights by using a much larger dataset than any previous studies of longevity," notes study co-author Catherine Ball, who is affiliated with Ancestry.

According to the team, previous estimates indicated that human lifespan heritability ranged between 15 and 30 percent.

After looking at a carefully selected set of family trees and relevant information collected from over 400 million people surveyed by Ancestry — most of whom were of European descent and based in the United States — the investigators identified a different story.

The scientists combined mathematical and statistical modeling and analyzed the data of relatives born throughout the 19th and early 20th centuries. They saw that siblings and first cousins showed the same heritability estimates that previous studies had given.

However, the researchers also saw that the lifespans of spouses were much more similar than those of siblings of different biological sexes. This, the team believes, may be due to the fact that spouses share environments and many lifestyle habits.

Yet the most puzzling finding was this: a person's siblings-in-law and first-cousins-in-law also had very similar lifespans, despite the fact that they were not related by blood with this individual and did not live with them.

So, what is the cause of this seemingly unlikely similarity in lifespans between a person and their in-law relatives? Following further analyses, the researchers concluded that it could be due to a concept called assortative mating.

"What assortative mating means here is that the factors that are important for lifespan tend to be very similar between mates," explains Ruby.

Essentially, when we look for a partner, we are likely to choose someone with whom we share very similar traits — and this includes those that probably affect lifespan.

So, when they accounted for the effects of assortative mating, the researchers concluded that lifespan heritability is somewhere around 7 percent, and possibly lower.

"We can potentially learn many things about the biology of aging from human genetics, but if the heritability of lifespan is low, it tempers our expectations about what types of things we can learn and how easy it will be."

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